1. Field of the Invention
The present invention relates to a silicon nitride sintered body having high strength and toughness and outstanding oxidation resistance which can be used as a high-temperature structural material.
2. Description of the Prior Art
The sintered body of silicon nitride (Si.sub.3 N.sub.4) finds use as a heat-resistant structural material for gas turbines, heat exchangers, high-temperature bearings, steel-making high-temperature rolls, and the like on account of its outstanding high-temperature strength, thermal shock resistance, and corrosion resistance.
Usually, Si.sub.3 N.sub.4 is sintered together with a sintering aid (which is an oxide such as MgO, MgAl.sub.2 O.sub.4, Al.sub.2 O.sub.3, and Y.sub.2 O.sub.3) because it is not easily sintered alone. It is considered that the sintering with a sintering aid is based on the liquid phase sintering mediated by the liquid phase which forms at the time of sintering. In many cases, the liquid phase remains as the glass phase in the sintered body after sintering, adversely affecting the high-temperature characteristics such as high-temperature strength and creep resistance.
Ceramic materials called sialon composed of Si.sub.3 N.sub.4 and a variety of elements forming a solid solution therein are attracting attention on account of their outstanding high-temperature characteristics. They are expected to find use as high-temperature structural materials. Particularly notable are .alpha.'-Si.sub.3 N.sub.4 (commonly called .alpha.-sialon) and .beta.'-Si.sub.3 N.sub.4 (commonly called .beta.-sialon). The former is represented by the general formula of M.sub.x (Si,Al).sub.12 (O,N).sub.16 (where 0&lt;x.ltoreq.2, and M denotes Li, Mg, Ca, and Y.) It is based on the .alpha.-Si.sub.3 N.sub.4 structure containing Al at the Si position, O at the N position, and other elements (such as Li, Mg, Ca, and Y) at the interstitial sites, which form an interstitial solid solution. The latter is represented by the general formula of Si.sub.6-y Al.sub.y O.sub.y N.sub.8-y (where 0&lt;y.ltoreq.4.2). It is based on the .beta.-Si.sub.3 N.sub.4 structure containing Al at the Si position and O at the N position, which form a solid solution.
The sintered body of .alpha.'-Si.sub.3 N.sub.4 alone or .beta.'-Si.sub.3 N.sub.4 alone is inferior in room temperature strength and toughness to other sintered bodies of Si.sub.3 N.sub.4. Sintered bodies composed of .alpha.'-Si.sub.3 N.sub.4 and .beta.'-Si.sub.3 N.sub.4 are being developed as explained in the following.
It was reported in J. Materials Sci. 14 (1979) p. 1749 that mixed powder of Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 -AlN gives, upon sintering, a sintered body having a single phase of .alpha.'-Si.sub.3 N.sub.4 or a mixed phase of .alpha.'-Si.sub.3 N.sub.4 and .beta.'-Si.sub.3 N.sub.4. Japanese Patent Laid-open No. 185484/1983 discloses a sintered body having a mixed phase of .alpha.'-Si.sub.3 N.sub.4 and .beta.'-Si.sub.3 N.sub.4 which is produced from .alpha.'-Si.sub.3 N.sub.4 powder and .beta.'-Si.sub.3 N.sub.4 powder. These two sintered bodies are poor in room-temperature strength and high-temperature strength.
In addition, Japanese Patent Laid-open No. 182276/1984 discloses a sintered body composed of .alpha.'-Si.sub.3 N.sub.4 and .beta.-Si.sub.3 N.sub.4 which is produced by sintering a mixed powder of Si.sub.3 N.sub.4 -AlN-Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3. It is claimed that this sintered body has an improved high-temperature strength if the ratio of .alpha.'-Si.sub.3 N.sub.4 present is 0.05 to 0.7 and the crystal grain size is smaller than 40 .mu.m measured in the direction of major axis. However, it is not satisfactory in high temperature strength at 1200.degree. C. and above.